The invention relates generally to the field of floating gate devices and more specifically to a programmable variable capacitor which incorporates a floating gate device.
Electronic devices perform several functions, including digital, analog and memory. Analog devices fall into many categories with one major category being that of frequency selective devices. Examples include voltage-controlled oscillators (VCO), narrow band tuned amplifiers and resonant tuning circuits. In general, frequency selectivity is performed by circuits comprising inductors and capacitors which are assembled in well known circuit topologies such as to exhibit frequency selective behavior. Examples include band-pass filters and input matching networks.
A key characteristic of resonant circuits is bandwidth, which is the frequency band over which the circuit passes a signal. Bandwidth is often described in both absolute terms (measured in Hz) and in relative terms (measured as a percentage of the center frequency). Both wide and narrow bandwidth circuits find widespread application in modern electronic communication systems. Communication systems are often categorized as wired or wireless, but the issues related to frequency selectivity are similar for both cases. In all cases, optimum performance is obtained when a circuit is tuned to ensure that its center frequency and bandwidth are matched to the center frequency and bandwidth of the application.
Many wired and most wireless communication systems (e.g. radios, a term used herein which is understood to refer not only to radios specifically, but to communication systems in general) are considered narrow band in that the entire allowed spectrum (e.g., in a cellular phone system) is typically no more than a few percent of the center frequency. In such systems, resonant circuits are typically tuned by mechanical techniques to align them to the broadcast frequency. Tuning is required because typical components and manufacturing techniques are generally not precise enough to achieve the desired alignment with the broadcast frequency accurately and inexpensively.
In many communication systems, it may also be necessary to adjust the frequency of individual devices. In the cellular phone example, multiple handsets operate within a single cell and it is often necessary that each phone operate at an assigned frequency (a so-called channel) that can vary from cell to cell and even from call to call. Such frequency agility is typical of wireless systems including AM/FM radios, television (both broadcast and cable), cell phones, pagers, mobile radios and virtually all other modern communications systems. It may also be desirable for a communication system to operate in multiple bands, which currently requires multiple tuned circuits. If a single circuit could be re-tuned, significant cost, weight and power consumption would be realized. These requirements for all forms of frequency agility place numerous requirements on the design and manufacture of the electronic devices performing communications functions.
Tuned circuits exhibit a response that is dependent on the frequency of an applied signal. The simplest tuned circuit is an L-C circuit, a circuit that is well known in the electronics industry. In the absence of any resistance, a pure L-C circuit would respond to a radian frequency of (1/LC)1/2, where L is the inductance and C is the capacitance. Hence doubling the value of the capacitance would reduce the center frequency by about 30%. This pure L-C circuit would also have an infinitely high Q. However, including resistance of the inductor, capacitor and wires of a non-ideal, i.e. real or physical, L-C circuit reduces the Q to values typically between 10 and 100.
Many variations on the tuned circuit theme have been used, including multiple components connected in an almost infinite number of topologies. Each topology has a characteristic response, but in general, their key features are center frequency, bandwidth and transition region. In each design, tradeoffs between efficiency (high Q) and bandwidth (typically wider for lower Q) must be tolerated and acceptable compromises determined. In general, a radio""s bandwidth is first determined by the system specification, then the highest Q components that are consistent with the system cost and specification are selected. However, since all components have manufacturing variations, tuned circuits usually require adjustment to get them to operate at their designed frequency.
Certain tuned circuits are designed to operate in a narrow segment (channel) of a system""s bandwidth. Such circuits are critically important to a radio""s performance since they must be much narrower than the overall system and they must be frequency agile. The most common such circuit is the aforementioned VCO. The element within the VCO that actually causes frequency shifting is a variable capacitor, also referred to as a varactor.
Frequency agility is usually provided by a circuit which changes frequency in response to an applied voltage, i.e., a circuit often referred to as a voltage controlled oscillator, or VCO. Typically, a VCO circuit includes a component referred to as a varactor (contraction of variable-capacitor) or varicap or voltacap, i.e., a capacitor which changes value in response to an applied voltage. The term varactor will be used herein to refer to all of these types of devices. Presently, many varactors are made from semiconductor materials such as silicon and utilize devices that typically include a p-n junction (e.g., a diode). These devices use the well-known effect that a diode""s depletion capacitance decreases as the D-C voltage applied across the p-n junction increases (when applied in a reverse bias condition). While such devices provide the variable capacitance required to adjust the tuning of a resonant circuit, they have numerous drawbacks, including relatively high resistance (hence a low quality factor, Q), large variations in their value of capacitance and large variations in their voltage sensitivity. Nonetheless, these devices are found in most modern radios.
Quality factor, Q, is a ratio of the capacitive effect to the resistive effect with high Q values being desirable. In diode varactors, it is necessary to use highly resistive material to form the variable capacitance, which in turn creates relatively high resistance. In this type of device, Q factors above 10 at 2 GHz are considered good, and are often listed as high-Q devices. Highly resistive material is also highly sensitive to variations in its processing conditions, which in turn causes large variations in the value of capacitance and the change in capacitance per unit applied voltage. In production, a typical high Q varactor design can exhibit a 30-50% variation in its capacitance values from component to component, even though the same materials and manufacturing processes are used to produce the individual components.
These variations in component value result in errors in the frequency of the VCO. These frequency errors are often greater than the entire bandwidth of the system; hence the radio operates incorrectly (and often in violation of license limits). To correct for this error and the combined errors of other critical components, most modern VCO""s are tuned to the correct frequency in a labor-intensive, expensive and often mechanical process. For example, it is often necessary to use mechanical tuning capacitors and laser-trimmed capacitors.
The present invention addresses the above described shortcomings in varactor design and production. The present invention provides a design and method for producing a superior varactor which can be electronically tuned and shipped with improved accuracy and which can be electronically tuned in the assembled circuit to permit for correction of errors due to other components in the circuit. When compared to currently available varactors, the varactor of the present invention exhibits higher Q factors and reduced variation in critical parameters. For example, while conventional diode varactors exhibit a Q of about 10 at 2 GHz, the varactor of the present invention exhibits a Q of from approximately 20-40 at 2 GHz, depending on the layout of the device. Additionally, while conventional diode varactors typically exhibit errors in the actual value of capacitance in the range of 30-50% as compared to the intended design value, the varactor of the present invention exhibits errors of less than approximately 5% from the intended design value.
In a first aspect, the present invention is a variable capacitor comprising: an insulating substrate; a first semiconductive region formed on the insulating substrate; a first electrode electrically coupled to the first semiconductive region; a first gate which is electrically floating and is capacitively coupled to the first semiconductive region, wherein a capacitance C1 represents the capacitive coupling between the floating first gate and the first semiconductive region; a conducting region capacitively coupled to the floating first gate, wherein a capacitance C2 represents the capacitive coupling between the conducting region and the floating first gate; and a second electrode electrically coupled to the conducting region. In some configurations, the insulating substrate further comprises sapphire. In some configurations, the variable capacitor may further comprise a second semiconductive region formed on the insulating substrate wherein the first semiconductive region is electrically coupled to the second semiconductive region which is electrically coupled to the first electrode. Additionally, the first semiconductive region may further comprise an N type semiconductor and the second semiconductive region may further comprise an N+ type semiconductor. Some configurations of the variable capacitor may further comprise at least one electrically insulating region which electrically insulates the first semiconductive region, the floating first gate and the conducting region from each other, wherein the first capacitance C1 between the floating first gate and the first semiconductive region may further comprise an insulator/oxide capacitance COX and a depletion capacitance CDEP wherein the depletion capacitance CDEP varies as a function of a voltage applied between the first and second electrodes. In some configurations, the variable capacitor exhibits a first total capacitance CT1 when a voltage V1 is applied between the first and second electrodes and a second total capacitance CT2 when a voltage V2 is applied between the first and second electrodes, wherein the difference between the first and second total capacitances, (CT2xe2x88x92CT1), is a function of the capacitance C2 between the conducting region and the floating first gate. The variable capacitor may further include a charge injector electrically coupled to the variable capacitor floating first gate, wherein the charge injector injects charge onto the variable capacitor floating gate. In some configurations, the charge injector further comprises: an island of semiconductor material on an insulating substrate wherein the island of semiconductor material further comprises: a first region and a second region of a first conductivity type separated by a channel region positioned between the first and second regions; and a third region of a second conductivity type which is adjacent to the channel region; and a charge injector floating gate positioned over the channel region wherein the charge injector floating gate is electrically coupled to the variable capacitor floating first gate and injects charge onto the variable capacitor floating gate. In some configurations, the variable capacitor exhibits: a first total capacitance CT1 when a voltage V1 is applied between the first and second electrodes and a second total capacitance CT2 when a voltage V2 is applied between the first and second electrodes; and a midpoint capacitance between capacitance CT1 and capacitance CT2 at a midpoint voltage VMID between voltage V1 and voltage V2, wherein the value of the midpoint voltage VMID is a function of the charge injected onto the variable capacitor floating gate from the charge injector.
In a second aspect, the present invention is a variable MOS capacitor comprising: a first semiconductive region; a first electrode electrically coupled to the first semiconductive region; a first gate which is electrically floating and is capacitively coupled to the first semiconductive region, wherein a capacitance C1 represents the capacitive coupling between the floating first gate and the first semiconductive region; a conducting region capacitively coupled to the floating first gate, wherein a capacitance C2 represents the capacitive coupling between the conducting region and the floating first gate; a second electrode electrically coupled to the conducting region; and a charge injector electrically coupled to the floating first gate for injecting charge onto the floating first gate. This variable MOS capacitor may further comprise an insulating substrate, wherein the first semiconductive region is formed on the insulating substrate. In some configurations, the insulating substrate further comprises sapphire. Some configurations of the variable MOS capacitor further comprise a second semiconductive region wherein the first semiconductive region is electrically coupled to the second semiconductive region which is electrically coupled to the first electrode. In some configurations, the first semiconductive region further comprises an N type semiconductor and the second semiconductive region further comprises an N+ type semiconductor.
In a third aspect, the present invention is a programmable MOS capacitor comprising: a first semiconductive region; an electrical contact in electrical contact with the first semiconductor region; a first gate which is electrically floating and is capacitively coupled to the first semiconductive region, wherein the first floating gate overlaps at least a portion of the first semiconductive region thereby enabling a depletion capacitance to be formed in the first semiconductive region; a first insulating region positioned between the first semiconductive region and the first floating gate; a conducting region capacitively coupled to the floating first gate; and a charge injector electrically coupled to the floating first gate for injecting charge onto the floating first gate. In some configurations, the programmable MOS capacitor further comprises an insulating substrate, wherein the first semiconductive region is formed on the insulating substrate. In some configurations, the insulating substrate further comprises sapphire. In some configurations, the programmable MOS capacitor further comprising a second semiconductive region wherein the first semiconductive region is electrically coupled to the second semiconductive region which is electrically coupled to the electrical contact. In some configurations, the first semiconductive region further comprises an N type semiconductor and the second semiconductive region further comprises an N+ type semiconductor.
In a fourth aspect, the present invention is a MOS capacitor comprising: a floating gate which overlaps at least a portion of a first semiconductive region wherein a depletion region is formed; and a charge injector electrically coupled to the floating gate for injecting charge onto the floating gate. The MOS capacitor may further comprise an insulating substrate, wherein the first semiconductive region is formed on the insulating substrate. In some configurations, the insulating substrate further comprises sapphire. In some configurations, the MOS capacitor further comprises a second semiconductive region wherein the first semiconductive region is electrically coupled to the second semiconductive region. In some configurations, the first semiconductive region further comprises an N type semiconductor and the second semiconductive region further comprises an N+ type semiconductor.
In a fifth aspect, the present invention is a method for modifying a C-V plot which is characteristic of a variable MOS capacitor comprising injecting charge onto a floating gate which overlaps at least a portion of a semiconductive region of the MOS capacitor wherein a depletion region is formed.